In-line optical waveguide refractive index gratings are periodic, aperiodic or pseudo-periodic variations in the refractive index of a waveguide. Gratings may be formed, for example, by physically impressing a modulation on the waveguide, by causing a variation of the refractive index along the waveguide using the photosensitivity phenomenon, or by other methods known in the art. In particular, gratings written into the core of an optical fiber are critical components for many applications in fiber-optic communication and sensor systems.
Dopants, such as germanium, are added to an area of the waveguide material to make it photosensitive, causing the refractive index of that region to be susceptible to increase upon exposure to actinic radiation. The currently preferred method of "writing" an in-line grating comprises exposing a portion of the waveguide to the interference between two beams of actinic (typically UV) radiation. The two beams are incident on the guiding structure of the waveguide in a transverse direction to create an interferogram, that is, a pattern of optical interference. The angle between the two beams (and the wavelength of the radiation) defines the fringe spacing of the interferogram. Typically, the two beams of actinic radiation are the legs of an interferometer or are produced by launching a single beam through a phase mask. The phase mask method is considered generally more suitable for large scale manufacture of in-line gratings, because it is highly repeatable, less susceptible to mechanical vibrations of the optical setup, and can be made with writing beams of much shorter coherence length.
Advantages of optical fiber in-line gratings over competing technologies include all-fiber geometry, low insertion loss, high return loss or extinction, and potentially low cost. But one of the most distinguishing features of fiber gratings is the flexibility the gratings offer for achieving desired spectral characteristics. Numerous physical parameters of the gratings can be varied, including induced index change, length, apodization, period chirp, grating pitch tilt, and whether the grating supports coupling into co-propagating (long-period or transmission gratings) or counter-propagating coupling (Bragg gratings) at a desired wavelength. By varying these parameters, gratings can be tailored for specific applications.
The versatility of an in-line grating is largely dependent on two factors, the overall length of the grating structure and the reflectivity (or transmission) profile of the grating structure itself Intricate reflectivity profiles can be achieved by carefully controlling the refractive index perturbation along the waveguide length, x. The index perturbation .differential.n(x) may be characterized as a phase and amplitude-modulated periodic function, ##EQU2## where .differential.n.sub.0 (x) is the "dc" index change spatially averaged over a grating period, A(z) is an offset (typically A=1), m(x) is the fringe visibility of the index change, .LAMBDA. is the nominal period and .phi.(x) describes grating chirp. To automate the fabrication process, it is desirable to write this arbitrary refractive index profile into a waveguide in a single process step, i.e., with a single pass of the laser beam over the waveguide and without physically changing the writing apparatus. For fill flexibility in grating manufacture, one needs to control independently each of the parameters describing .differential.n(x).
In particular, apodization of a grating spectrum may be achieved by controlling say .differential.n.sub.0 (x) and m(x) along the grating length. The main peak in the reflection spectrum of a finite length in-line grating with uniform modulation of the index of refraction is accompanied by a series of sidelobes at adjacent wavelengths. Lowering the reflectivity of the sidelobes, or "apodizing" the reflection spectrum of the grating, is desirable in devices where high rejection of nonresonant light is required. Apodization also improves the dispersion compensation characteristics of chirped gratings. In most of these applications, one desires apodization created by keeping .differential.n.sub.0 (x) and A(x) constant across the grating length while m(x) is varied, which is believed not to have been achieved (with full flexibility) in a single-step process by controlling only the laser beam.
Variation of the index modulation by changing the ultraviolet exposure along the length of the grating causes both the magnitude of the refractive index modulation and the average photoinduced refractive index to vary. The average index variation leads to undesirable effective chirps of the resonant wavelength of the grating and widens the grating spectral response. To alleviate these symptoms, it is desirable to "pure apodize" the grating, that is, to generate both the non-uniform modulated ultraviolet fringe pattern and a compensating exposure which automatically ensures that the average photoinduced refractive index is constant along the length of the fiber. Some researchers have created the desired apodization profile by dithering the waveguide in the interferogram to decrease refractive index fringe visibility at specified locations along the waveguide length, but these techniques require complex mechanical fixtures for the phase mask and waveguide that can be vibrated yet precisely positioned.
In addition to the specific index perturbation written into the waveguide, grating length is also important in certain applications in optical fiber communication and distributed sensor systems. For instance, long-length chirped fiber Bragg gratings have been suggested as attractive devices for the manufacture of dispersion compensators. High-speed, long distance data transmissions, especially transmissions over existing non-dispersion shifted fiber networks, are limited by chromatic dispersion in the optical fiber. Since the transmission bandwidth usually is predetermined by the needs of the system, to be usable as dispersion compensators in practice, chirped Bragg gratings need to exhibit dispersion compensation over a bandwidth large enough to cover typical semiconductor laser wavelength tolerances. It has been reported that a grating of the order of 1 meter in length with a constant dispersion profile and a broad bandwidth would be required to achieve a time delay of .about.1700 ps/nm sufficient to compensate for 100 km of non-dispersion shifted fiber over 5 nm at a wavelength of 1550 nm.
The need exists for a method for producing long length Bragg gratings having complex grating structures. One method has been described where a UV-beam is scanned over a long phase mask having a fixed position relative to the fiber. Complex structures are added by varying the exposure time or by postprocessing the grating. Another method discusses the use of fibers held in a fixed position relative to specially designed long phase masks having the complex structure already imprinted in the mask. However, both of these techniques are limited by the length of available phase masks, usually about .about.10 cm.
A method for writing gratings where the waveguide moves in relation to the mask has been suggested. However, this technique is limited, since the fringe visibility of the index modulation in the waveguide will decrease significantly if the waveguide moves relative to the phase mask too much, so gratings much larger than a phase mask cannot be made. Recent developments have attempted to produce long complex gratings by scanning a UV-beam over a phase mask and writing sub-gratings (a number of grating elements) at every irradiation step on the fiber while moving the fiber using a very precise piezoelectric transducer. To increase the size of the grating structure, a number of subgratings may then be concatenated to one another. The fiber is translated with high-precision staging relative to an interferogram of UV-light. The position of the stage is tracked interferometrically and the laser is triggered when the fiber reaches the desired position for the next irradiation. The phasing between these subgratings may be controlled to create some complex structures, such as chirps. Apodization may be achieved by dithering about an interferogram/fiber relative position.
The concatenation process suffers from needing extremely accurate positioning staging, which is currently available only by using an interferometer as an encoder. Without interferometric control, the concatenation methods suffers from "stitching" errors, i.e., errors in the matching of the grating elements. Presently only linear motion staging can be interferometrically controlled; rotary stages must use mechanically-ruled encoders. Therefore, the length of a fiber grating made with a concatenation process is limited by the linear travel available on precision stages, the implementation of which currently become prohibitively expensive if much longer than one meter. Since the protective housing around a fiber must be removed for grating fabrication, a long length of bare fiber containing the grating is removed from the precision staging and coiled for packaging, which increases fabrication complexity (increased handling), complicates manufacture automation, and is likely to reduce the mechanical strength of the fiber.
The need remains for an effective writing technique for very long length in-line optical waveguide gratings having complicated reflectivity profiles.